From the prior art, lids for a packaging container are known to be produced by a process called IML (In-Mould Labelling, or Inner Mould Labelling). This method comprises depositing within a mold, a panel forming a label. The panel is positioned so that it may appear on the outer face of the finished product.
Thereafter, a plastic material for forming the support means of the panel and the body of the lid is heated at an elevated temperature and pressure, and injected into the mold. The panel is then secured to the plastic material to form the finished product.
Such a process, even while offering a perfect finish of the label, consumes a considerable amount of plastic, resulting in extra costs for the operator.
In order to overcome this drawback, it has been proposed to reduce the amount of plastic injected into the central part of the panel. However, under these conditions, the panel tends to wrinkle, making the inscriptions printed on it unreadable.
Also known from the document FR 2 827 841 is a lid for a packaging container composed of an annular member and a plastic panel. The annular member comprises an annular ridge arranged to cooperate with a complimentary ridge located on the container, so as to ensure the fixation of the lid to the container.
The annular member also comprises an annular projection extending radially and defining the periphery of an opening. This annular projection has an inner face intended to be turned towards the container, and an outer face opposite the inner face.
The panel comprises an inner face intended to be turned towards the container, and an outer face, printed opposite to the inner face.
During the manufacture of the lid, the annular member is overmolded onto the peripheral edge of the panel so that the outer face of the panel is secured in abutment against the inner face of the annular projection of the annular member.
Such a lid is disadvantageous in that, during overmolding of the annular member on the peripheral edge of the panel, the plastic material intended to form the annular member is brought to elevated temperature and pressure (e.g., 220° at 900 bar pressure) and then injected into the mold, resulting in the presence of thermal and mechanical stresses on the peripheral edge of the panel. When said injected plastic cools, the panel tends to retract.
Furthermore, it is common that the panel creases and as such, the printing performed on the outside of the front panel prior to the overmolding becomes unreadable after the overmolding or has an non-esthetic finish. This results in high scrap rates.
To overcome this drawback, it is known to increase the thickness of the panel in order to limit its contraction during the cooling of the injected material forming the annular member.
However, this results in an increase of the amount of plastic material used, and thus of the manufacturing cost of the lid.
Such a lid is also disadvantageous in that when pressure is exerted on the outer face of the panel, for example, when stacking a plurality of packaging containers, the panel tends to separate from the annular member, and a gap may appear between the panel and the annular member, which is detrimental to the airtightness of the lid.